Microphone with aligned apertures

ABSTRACT

A MEMS microphone has a backplate with a given backplate aperture, and a diaphragm having a diaphragm aperture. The given backplate aperture is substantially aligned with the diaphragm aperture.

PRIORITY

This application is a divisional application of U.S. patent applicationSer. No. 12/133,599, filed on Jun. 5, 2008, by Eric Langlois, et al.,entitled, “Microphone with Aligned Apertures”, which claims priorityfrom provisional U.S. patent application No. 60/942,315, filed Jun. 6,2007, entitled, “MICROPHONE WITH ALIGNED APERTURES,” and naming EricLanglois, Thomas Chen, Xin Zhang, and Kieran Harney as joint inventors,the disclosure of which is incorporated herein, in its entirety, byreference.

FIELD OF THE INVENTION

The invention generally relates to microphones and, more particularly,the invention relates to controlling the low frequency cutoff point formicrophone.

BACKGROUND OF THE INVENTION

Condenser microphones generally have a movable diaphragm that vibratesto produce a signal representative of an incident audio signal. Toensure that audio signals contact their respective diaphragms, prior artcondenser microphones known to the inventors have apertures in theirbackplates directly under a solid portion of the diaphragm. Accordingly,audio signals pass through the backplate apertures to directly contactthe diaphragm.

Condenser microphones typically are responsive to audio signals havingfrequencies that are greater than a predetermined low frequency cutoffpoint. This low frequency cutoff point often is set by controlling theresistance of the air flowing past the microphone diaphragm. Thisresistance, however, can be relatively high due to the positioning ofthe apertures directly under a solid portion of the diaphragm.Undesirably, setting the low frequency cutoff can be difficult due tosuch high resistance.

One method of controlling this low frequency cutoff point/resistancevaries the gap formed between the diaphragm and the stationary supportstructure supporting the diaphragm. For example, the gap may be enlargedto raise the cutoff point, or reduced to lower the cutoff point. Such amethod, however, has drawbacks. Among other things, it dictates the gapsize in a manner that may interfere with other design considerations.

In addition, controlling the gap size often does not sufficientlyaddress the above noted air resistance problem, in which the backplateaperture is directly under a solid portion of the diaphragm.Specifically, a portion of the sound wave path must be generallyhorizontal to reach the diaphragm gap. As such, controlling the gap sizeprovides relatively coarse control of the cutoff point. Electronic orother non-mechanical means then may be required to sufficiently tune thecutoff point of the microphone.

SUMMARY OF THE INVENTION

In accordance with one embodiment of the invention, a MEMS microphonehas a backplate with a given backplate aperture, and a diaphragm havinga diaphragm aperture. The given backplate aperture is substantiallyaligned with the diaphragm aperture.

For example, the given backplate aperture is not offset from thediaphragm aperture. The given backplate may form any of a number ofshapes, such as a slot or generally round opening. In a similar manner,the diaphragm aperture also may form any of a number of differentshapes, such as a slot.

The backplate may be generally parallel with and spaced in a verticaldirection from the diaphragm, while the given backplate aperture may besubstantially aligned with the diaphragm aperture in the verticaldirection. The MEMS microphone also may have a plurality of springscoupling the diaphragm to a substrate. As such, the plurality of springsmay define (at least in part) the diaphragm aperture.

The backplate may have first and second sets of backplate apertures. Thegiven backplate aperture may be in the first set, while the second setof backplate apertures may be offset from the diaphragm aperture.

In accordance with another embodiment of the invention, a MEMSmicrophone has a stationary support, a movable diaphragm, and aplurality of springs movably connecting the diaphragm to the stationarysupport. The microphone also has a backplate, with a plurality ofapertures, that is spaced from the diaphragm. The stationary support,diaphragm, and springs form a plurality of diaphragm apertures, while afirst diaphragm aperture is at least partially aligned with a firstbackplate aperture.

In this and other embodiments, the backplate may have another backplateaperture that is not aligned with (i.e., it is offset from) the firstdiaphragm aperture.

In accordance with other embodiments of the invention, a method offorming a MEMS microphone provides a backplate, forms a diaphragm spacedfrom the backplate, and forms a plurality of backplate apertures. Thediaphragm forms a diaphragm aperture, and a given aperture of thebackplate apertures is at least partially aligned with the diaphragmaperture.

The diaphragm may be formed by depositing a deposition material onto asacrificial material supported by the backplate, forming a plurality ofsprings, and removing the sacrificial material. The plurality of springssuspend the diaphragm so that it is vertically spaced from thebackplate. Among other things the backplate may be formed from an SOIwafer (i.e., a silicon-on-insulator wafer).

BRIEF DESCRIPTION OF THE DRAWINGS

Those skilled in the art should more fully appreciate advantages ofvarious embodiments of the invention from the following “Description ofIllustrative Embodiments,” discussed with reference to the drawingssummarized immediately below.

FIG. 1 schematically shows a mobile telephone that may use a MEMSmicrophone configured in accordance with illustrative embodiments of theinvention.

FIG. 2 schematically shows a MEMS microphone that may be configured inaccordance with illustrative embodiments of the invention.

FIG. 3 schematically shows a cross-sectional view of the microphoneshown in FIG. 1 across line 2-2.

FIGS. 4A and 4B schematically show plan views of two different backplatedesigns that may be used in accordance with illustrative embodiments ofthe invention.

FIG. 5 schematically shows a plan view of a diaphragm that may be usedin accordance with illustrative embodiments of the invention.

FIGS. 6A and 6B show a process of forming a microphone that is similarto the microphone 18 shown in FIGS. 2 and 3 in accordance withillustrative embodiments of the invention.

FIGS. 7A-7G schematically show the microphone of FIG. 2 during variousstages of fabrication using the process of FIG. 6.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

In illustrative embodiments, the diaphragm and backplate of a MEMSmicrophone cooperate to reduce air resistance through the microphone. Asa result, the microphone can be more easily tuned to a precise lowfrequency cutoff point. Details of illustrative embodiments arediscussed below.

FIG. 1 schematically shows a mobile telephone 10 that can use amicrophone configured in accordance with illustrative embodiments. Insimplified terms, the telephone 10 has a receiver 12 for receiving anaudio signal (e.g., a person's voice), a speaker portion 14 forgenerating audio signals, and a transponder 16 for transmitting andreceiving electromagnetic signals encoding audio signals. During use, aperson may speak into the receiver 12, which has a microphone (FIG. 2,discussed below) that converts the person's voice into an electricalsignal. Internal logic (not shown) and the transponder 16 modulate thissignal to a remote source, such as to a satellite tower and, ultimately,to another person on another telephone 10.

In illustrative embodiments, the receiver 12 has a microphonemechanically configured with a relatively precise low frequency cutoffpoint (i.e., the lowest frequency that it can detect without significantdistortion—often referred to in the art as the “3 dB point”). FIG. 2schematically shows a top, perspective view of a MEMS microphone 18(also referred to as a “microphone chip 18”) that may be fabricated inthis manner; namely, in accordance with illustrative embodiments of theinvention. FIG. 3 schematically shows a cross-sectional view of the samemicrophone 18 across line 2-2 of FIG. 2.

Among other things, the microphone 18 includes a static backplate 20that supports and forms a variable capacitor with a flexible diaphragm22. In illustrative embodiments, the backplate 20 is formed from singlecrystal silicon (e.g., the top layer of a silicon-on-insulator wafer,discussed below), while the diaphragm 22 is formed from a depositedmaterial, such as deposited polysilicon. Other embodiments, however, useother types of materials to form the backplate 20 and the diaphragm 22.For example, a single crystal silicon bulk wafer, or some depositedmaterial, may form the backplate 20. In a similar manner, a singlecrystal silicon bulk wafer, part of a silicon-on-insulator wafer, orsome other deposited material may form the diaphragm 22.

To facilitate operation, the backplate 20 has a plurality ofthrough-hole apertures (“backplate apertures 24”) that lead to abackside cavity 26. FIGS. 4A and 4B schematically show plan views of twodifferent backplates 20 that each have different configurations ofbackplate apertures 24. One such configuration has both generally roundholes and slots (i.e., elongated holes), while the other configurationhas only generally round holes arranged in a specific pattern. Itnevertheless should be noted that the patterns and types of apertures 24shown in FIGS. 4A and 4B are just illustrative and not intended to limitall embodiments of the invention. Various embodiments thus may employother configurations of apertures 24.

Springs 28 movably connect the diaphragm 22 to a static/stationaryportion 30 of the microphone 18, which includes a substrate (alsoidentified by reference number “30”). The springs 28 effectively form aplurality of apertures that permit at least a portion of the audiosignal to pass through the diaphragm 22. These apertures 32, which alsoare referred to as “diaphragm apertures 32,” may be any reasonableshape, such as in the shape of a slot, round hole, or some irregularshape.

More specifically, FIG. 5 schematically shows a diaphragm 22 that may beused in accordance with illustrative embodiments of the invention. Asshown, the diaphragm 22 has four springs 28 that suspend it to begenerally parallel to and above the backplate 20. In other words, fromthe perspective of FIG. 3, the diaphragm 22 may be considered to bevertically spaced from the backplate 20. With reference to the diaphragm22 shown in FIG. 5, the following portions of the microphone 18effectively form each noted diaphragm aperture 32:

1) each adjacent pair of springs 28,

2) the stationary portion 30 immediately adjacent to and between thespring pairs, and

3) the corresponding diaphragm edge 34 between the pair of springs 28.

For example, the apertures 32 shown in FIG. 5 effectively are slots.

Other embodiments, however, may have other types of springs 28 andapertures 24 and 32. For example, the springs 28 may have a serpentineshape, such as that disclosed in co-pending U.S. patent application Ser.No. 12/015,903, filed Jan. 17, 2008, and attorney docket number2550/B81, the disclosure of which is incorporated herein, in itsentirety, by reference. In that patent application, the aperturesthrough the diaphragm have shapes corresponding to the serpentine natureof the springs.

Incident audio signals cause the diaphragm 22 to vibrate, thus producinga changing capacitance between it and the backplate 20. Such audiosignals may contact the microphone 18 from any direction. For example,in FIG. 3, the audio signals are shown as traveling upwardly, firstthrough the backplate 20, and then partially through and against thediaphragm 22. In other embodiments, the audio signals may travel in theopposite direction. On-chip or off-chip circuitry (not shown) receive(via contacts 36 of FIG. 2) and convert this changing capacitance intoelectrical signals that can be further processed.

It should be noted that discussion of the specific microphone 18 shownin FIGS. 2-5 is for illustrative purposes only. Other microphoneconfigurations thus may be used consistent with various embodiments ofthe invention.

In accordance with illustrative embodiments of the invention, thebackplate apertures 24 are substantially aligned with the diaphragmapertures 32. This is in contrast to prior art designs known to theinventors, which offset the vertical alignment of the backplateapertures 24 and diaphragm apertures 32.

Accordingly, as shown in FIG. 3, at least a portion of an incident audiosignal can traverse substantially straight through the microphone 18.Such alignment therefore reduces the air resistance through themicrophone 18 because a portion of such audio signals does not travel ina direction that is generally parallel to the plane of the diaphragm 22.

In some embodiments, the diaphragm apertures 32 are substantiallyexactly aligned with the apertures 24 through the backplate 20 (e.g.,see FIG. 3). Other embodiments, however, may only partially align thediaphragm apertures 32 and the backplate apertures 24. Moreover, inillustrative embodiments, the aligned backplate apertures 24 aresubstantially the same shape and area as that of the diaphragm apertures32. Alternative embodiments, however, do not have such a requirement.

One backplate aperture 24 may at least partially align with one or morediaphragm apertures 32. In a corresponding manner, one diaphragmaperture 32 may at least partially align with one or more backplateapertures 24. Those skilled in the art can use other alignmentconfigurations within the spirit of various embodiments. Theseconfigurations may be useful with microphones having serpentine springs.Specifically, microphones having serpentine springs may be considered toform a plurality of regularly or irregularly shaped diaphragm apertures32. For example, some of those diaphragm apertures 32 may be spacedradially from each other, and/or along the general circumference of thediaphragm 22.

As shown in FIGS. 4A and 4B, the backplate 20 may be considered to haveat least two sets of backplate apertures 24; namely, a first set that isnot aligned with any diaphragm apertures 32 (i.e., they are offset fromthe diaphragm apertures 32), and a second set that is substantiallyaligned with diaphragm apertures 32. The second set shown in FIGS. 4Aand 4B are radially outwardly positioned from the first set. Despitethat, some embodiments may have additional diaphragm apertures 32 thatare not formed by the springs 28, stationary portion 30, and edge 34 ofthe diaphragm 22. Instead, these diaphragm apertures 32 may be formed byholes through the diaphragm 22 and may have, or may not have,corresponding aligned backplate apertures 24. In fact, some embodimentshave both types of diaphragm apertures 32.

As noted above, the inventors discovered that alignment of the diaphragmand backplate apertures 32 and 24, or even partial alignment, enabledthem to more precisely tune the low frequency cutoff point while stillmaintaining relatively thin diaphragm apertures 32. For example, thislow frequency cutoff point may be set to between about 50 and 100 Hertzwithout requiring use of filtering electronics. This is contrary to theinventors' understanding of the prior art, which preferred offsetapertures to ensure more of the signal contacted the diaphragm. Thus,contrary to what they understood to be the conventional wisdom, theinventors determined that the resulting signal loss, if any, due toaperture alignment was negligible. Accordingly, since such loss wasnegligible, the inventors were able to deviate from the prior artpractice of intentionally misaligning the noted apertures.

This alignment also provides some stress relief in overpressure events.Specifically, by reducing the air resistance through the microphone 18,this alignment permits air pressure to pass more freely through themicrophone 18. As a result, the springs 28 are less stressed and,consequently, less likely to fracture during overpressure events.

FIGS. 6A and 6B show a process of forming a microphone that is similarto the microphone 18 shown in FIGS. 2 and 3 in accordance withillustrative embodiments of the invention. The remaining figures (FIGS.7A-7G) illustrate various steps of this process. It should be noted thatthis process does not describe all steps required for forming themicrophone 18. Instead, it shows various relevant steps for forming themicrophone 18. Accordingly, some steps are not discussed.

The process begins at step 600, which etches trenches 38 in the toplayer of a silicon-on-insulator wafer (“SOI wafer 40”). These trenches38 ultimately form the backplate apertures 24—some of which are alignedin the manner discussed above with the yet-to-be-formed diaphragmapertures 32.

Next, the process adds sacrificial oxide 42 to the walls of the trenches38 and along at least a portion of the top surface of the top layer ofthe SOI wafer 40 (step 602). Among other ways, this oxide 42 may begrown or deposited. FIG. 7A schematically shows the wafer at this pointin the process. Step 602 continues by adding sacrificial polysilicon 44to the oxide lined trenches 38 and top-side oxide 42.

After adding the sacrificial polysilicon 44, the process etches a hole46 into the sacrificial polysilicon 44 (step 604, see FIG. 7B). Theprocess then continues to step 606, which adds more oxide 42 tosubstantially encapsulate the sacrificial polysilicon 44. In a mannersimilar to other steps that add oxide 42, this oxide 42 essentiallyintegrates with other oxides it contacts. Step 606 continues by addingan additional polysilicon layer that ultimately forms the diaphragm 22(see FIG. 7C). This layer illustratively is patterned to substantiallyalign at least some of the diaphragm apertures 32 with some of thebackplate apertures 24 in the manner discussed above.

Nitride 48 for passivation and metal for electrical connectivity alsoare added (see FIG. 7D). For example, deposited metal may be patternedto form a first electrode 50A for placing electrical charge on thediaphragm 22, another electrode 50B for placing electrical charge on thebackplate 20, and the contacts 36 for providing additional electricalconnections.

The process then both exposes the diaphragm 22, and etches holes throughthe diaphragm 22 (step 608). As discussed below in greater detail, oneof these holes (“diaphragm hole 52A”) ultimately assists in forming apedestal 54 that, for a limited time during this process, supports thediaphragm 22. A photoresist layer 56 then is added, completely coveringthe diaphragm 22 (step 610). This photoresist layer 56 serves thefunction of an etch mask.

After adding the photoresist 36, the process exposes the diaphragm hole52A (step 612). To that end, the process forms a hole (“resist hole 58”)through the photoresist 36 by exposing that selected portion to light(FIG. 7E). This resist hole 58 illustratively has a larger innerdiameter than that of the diaphragm hole 52A.

After forming the resist hole 58, the process forms a hole 60 throughthe oxide 42 (step 614). In illustrative embodiments, this oxide hole 60effectively forms an internal channel that extends to the top surface ofthe SOI wafer 40.

It is expected that the oxide hole 60 initially will have an innerdiameter that is substantially equal to the inner diameter of thediaphragm hole 52A. A second step, such as an aqueous HF etch, may beused to enlarge the inner diameter of the oxide hole 60 to be greaterthan the inner diameter of the diaphragm hole 52A. This enlarged oxidehole diameter essentially exposes a portion of the bottom side of thediaphragm 22. In other words, at this point in the process, the channelforms an air space between the bottom side of the diaphragm 22 and thetop surface of the backplate 20.

Also at this point in the process, the entire photoresist layer 56 maybe removed to permit further processing. For example, the process maypattern the diaphragm 22, thus necessitating removal of the existingphotoresist layer 56 (i.e., the mask formed by the photoresist layer56). Other embodiments, however, do not remove this photoresist layer 56until step 622 (discussed below).

The process then continues to step 616, which adds more photoresist 36,to substantially fill the oxide and diaphragm holes 40 and 34 (FIG. 7F).The photoresist 36 filling the oxide hole 60 contacts the silicon of thetop SOI layer, as well as the underside of the diaphragm 22 around thediaphragm hole 52A.

The embodiment that does not remove the original mask thus applies asufficient amount of photoresist 36 in two steps (i.e., first the mask,then the additional resist to substantially fill the oxide hole 60),while the embodiment that removes the original mask applies a sufficientamount of photoresist 36 in a single step. In both embodiments, as shownin FIG. 7F, the photoresist 36 essentially acts as the single,substantially contiguous apparatus above and below the diaphragm 22.Neither embodiment patterns the photoresist 36 before the sacrificiallayer is etched (i.e., removal of the sacrificial oxide 42 andpolysilicon 44, discussed below).

In addition, the process may form the backside cavity 26 at this time.To that end, as shown in FIG. 7F, conventional processes may applyanother photoresist mask on the bottom side of the SOI wafer 40 to etchaway a portion of the bottom SOI silicon layer. This should expose aportion of the oxide layer within the SOI wafer 40. A portion of theexposed oxide layer then is removed to expose the remainder of thesacrificial materials, including the sacrificial polysilicon 44.

At this point, the sacrificial materials may be removed. To that end,the process removes the sacrificial polysilicon 44 (step 618) and thenthe sacrificial oxide 42 (step 620, FIG. 7G). Among other ways,illustrative embodiments remove the polysilicon 44 with a dry etchprocess (e.g., using xenon difluoride) through the backside cavity 26.In addition, illustrative embodiments remove the oxide 42 with a wetetch process (e.g., by placing the apparatus in an acid bath for apredetermined amount of time). Some embodiments, however, do not removeall of the sacrificial material. For example, such embodiments may notremove portions of the oxide 42. In that case, the oxide 42 may impactcapacitance.

As shown in FIG. 7G, the photoresist 36 between the diaphragm 22 and topSOI layer supports the diaphragm 22. In other words, the photoresist 36at that location forms a pedestal 54 that supports the diaphragm 22. Asknown by those skilled in the art, the photoresist 36 is substantiallyresistant to wet etch processes (e.g., aqueous HF process, such as thosediscussed above). It nevertheless should be noted that other wet etchresistant materials may be used. Discussion of photoresist 36 thus isillustrative and not intended to limit the scope of all embodiments.

Stated another way, a portion of the photoresist 36 is within the priornoted air space between the diaphragm 22 and the backplate 20; namely,it interrupts or otherwise forms a part of the boundary of the airspace. In addition, as shown in the figures, this photoresist 36 extendsas a substantially contiguous apparatus through the hole 52 in thediaphragm 22 and on the top surface of the diaphragm 22. It is notpatterned before removing at least a portion of the sacrificial layers.No patterning steps are required to effectively fabricate the microphone18.

To release the diaphragm 22, the process continues to step 622, whichremoves the photoresist 36/pedestal 54 in a single step. Among otherways, dry etch processes through the backside cavity 26 may be used toaccomplish this step. This step illustratively removes substantially allof the photoresist 36—not simply selected portions of the photoresist36.

It should be noted that a plurality of pedestals 42 may be used tominimize the risk of stiction between the backplate 20 and the diaphragm22. The number of pedestals used is a function of a number of factors,including the type of wet etch resistant material used, the size andshape of the pedestals 42, and the size, shape, and composition of thediaphragm 22. Discussion of a single pedestal 54 therefore is forillustrative purposes.

Accordingly, illustrative embodiments at least partially align thediaphragm and backplate apertures 32 and 24 to more precisely set thelow frequency cutoff.

Although the above discussion discloses various exemplary embodiments ofthe invention, it should be apparent that those skilled in the art canmake various modifications that will achieve some of the advantages ofthe invention without departing from the true scope of the invention.

What is claimed is:
 1. A method of forming a MEMS microphone, the methodcomprising: providing a backplate; forming a diaphragm spaced from thebackplate, the diaphragm forming a diaphragm aperture; and forming aplurality of backplate apertures, a given aperture of the backplateapertures being at least partially aligned with the diaphragm aperture.2. The method as defined by claim 1 wherein the given aperture issubstantially aligned with the diaphragm aperture.
 3. The method asdefined by claim 1 wherein the plurality of backplate apertures includesa second aperture that is offset from the diaphragm aperture.
 4. Themethod as defined by claim 1 wherein forming a diaphragm comprises:depositing a deposition material onto a sacrificial material supportedby the backplate; forming a plurality of springs; and removing thesacrificial material to form the diaphragm, the plurality of springssuspending the diaphragm, the diaphragm being vertically spaced from thebackplate.
 5. The method as defined by claim 1 wherein providing abackplate comprises forming the backplate from an SOI wafer.
 6. Themethod as defined by claim 1 wherein forming a plurality of backplateapertures includes etching a slot through a wafer.